This mission is given to you by the FIB, which is marked as "B" on your map. You begin the mission playing as Michael, and are joined by Steve Haines and Dave Norton. Drive down the coast and to the lab, where you will need to dive into the water. Follow both Steve and Dave through the water and you'll come to a metal grating at the entrance.
Here, you'll need to use the torch to go around the outside of the grating and turn all of the green sections to red. The only problem is that your torch only has a limited amount of fuel. To easily get past this part, line up the torch with the green areas, then use the torch on it to make it red (it will make a sound when it does). You do not want to keep the torch button held down, as this will waste fuel.
With the path now open, follow the other two inside the facility until you finally come to the surface. Climb the ladder and continue into the facility, encountering your first scientist in the next room. For now, you are equipped with the Stun Gun and must use it on anyone you come across for now. It must recharge back to 100% before you can fire it again, though, although you don't need to quickly use it for the first section.
Keep following the other two, tasering whoever you come across. When you get to the elevator, stun gun the guy and pick up his security card. You will come upon some security guards who are patrolling, but you can just let them pass. You'll eventually come to a part where the scientist won't open the door, but another comes along to entice him to open it. Tase them both and then go grab the nerve toxin.
You can now switch to a normal gun for the rest of the mission, as you will go from room to room, encountering plenty of security guards. Once you reach the outside area, place the nerve toxin in the cooling area and the game will switch to Trevor. Fly to the lab and collect the container (you simply need to get close to it). When the scene is over, you need to head to the airfield.
As you get close to the airfield, start to slow down, as the chopper you're controlling can be slightly hard to control in smaller doses. When you get there, you'll have to release the crate and make it land on the trailer, which can be a difficult process using the chopper. Once you do, land the chopper nearby and after the scene is over, you'll switch to Trevor. All you have to do here is drive Patricia back to Martin Madrazo to finish the mission.